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Observations of transits of the hot giant exoplanet HD 189733b in theunresolved HI Lyman-alpha line show signs of hydrogen escaping the upperatmosphere of the planet. New resolved Lyman-alpha observations obtained withthe STIS spectrograph onboard the Hubble Space Telescope in April 2010 andSeptember 2011 confirmed that the planet is evaporating, and furthermorediscovered significant temporal variations in the physical conditions of itsevaporating atmosphere. Here we present a detailed analysis of the September2011 observations of HD 189733b, when an atmospheric signature was detected. Wepresent specific methods to find and characterize this absorption signature ofescaping hydrogen in the Lyman-alpha line, and to calculate its false-positiveprobability, found to be 3.6%. Taking advantage of the spectral resolution andhigh sensitivity of the STIS spectrograph, we also present new results ontemporal and spectro-temporal variability of this absorption feature. We alsoreport the observation of HD 189733b in other lines (SiIII at 1206.5A, NV at1240A). Variations in these lines could be explained either by earlyoccultation by a bow-shock rich in highly ionized species, or by stellarvariations.

Atmospheric escape from HD 189733b observed in HI Lyman-alpha: detailed analysis of HST/STIS September 2011 observations